Image: Mars' Moon Phobos As Seen By Mars Express

This picture of Phobos near the limb of Mars was captured in 2010 by Mars Express currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side.

From images like this, Phobos has been determined to be covered by perhaps a meter of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, but from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years. Credit: G. Neukum (FU Berlin) et al., Mars Express, DLR, ESA; Acknowledgement: Peter Masek. Source: NASA Astronomy Picture of the Day